The invention relates to a negative radiation-curable mixture containing a compound which forms an acid under the action of high-energy radiation, and an acid-curable compound.
In classical UV lithography, the resolution limit is determined by the wavelength of the radiation used. The constant decrease in dimensions in chip production therefore requires new lithographic techniques in the submicron region, electron or X-ray radiation being employed due to its extremely short wavelength. At the same time, it has been shown that resist materials which are suitable as electron beam resist can also be employed as X-ray resist, and vice versa.
Known resist materials for this application are acrylates and methacrylates (G. M. Taylor, Solid State Technology, 124 (1984)). In the case of these materials, it has been shown that sensitivity and structural resolution are usually contrary properties. If it is to be possible for higher sensitivities to be achieved, halogens are usually incorporated into the resist. In this case, fluorine and chlorine are usually employed in positive resists, whereas it is usually bromine and iodine, besides chlorine, which are employed in negative resists (T. Yamaoka et al., Phot. Sci. Eng. 23, 196 (1979)).
In general, negative, i.e. radiation-curable, resists exhibit higher sensitivity than positive resists, but, in contrast, cannot simultaneously--as stated above--have a high resolution in the submicron region. On the other hand, positive methacrylate-based resists achieve high resolution, but, with the exception of polymethacrylonitrile-based resists, are not stable to the plasma etching processes used for semiconductor structuring. In turn, however, the methacrylate resists are not sufficiently sensitive.
The polymers having the highest radiation sensitivity known to date to electron beams or X-rays are polyalkene sulfones, in particular polybutene 1-sulfone. The disadvantage of this class of compounds is, however, that they are less resistant to plasma etching processes; they are therefore suitable for mask production, but not for semiconductor fabrication using a mask made of this material. It has therefore been proposed to combine polyalkene sulfones with novolak resins, which, as is generally known, are resistant to plasma etching (M. J. Bowden et al., J. Electrochem. Soc. 128, 1304 (1981); U.S. Pat. No. 4,289,845). However, it became apparent that the two polymers are extremely incompatible with one another, thus impairing the resolution. An attempt to improve the compatibility by admixing further components also had to pay the price of loss in sensitivity (U.S. Pat. No. 4,398,001).
In order to retain high sensitivities at the same time as an improved overall property profile, in particular improved plasma-etching resistance, photocatalytic resists have been developed. Examples of positive systems of this type are described in, inter alia, DE-A-2,718,254 (=U.S. Pat. No. 4,247,611) and 2,928,636 (=U.S. Pat. No. 4,311,782), and the earlier German Patent Applications P 37 30 784.3 and P 38 21 585.3.
Corresponding negative systems are, for example, resists which dimerize with crosslinking on irradiation, such as cinnamic acid derivatives, which are polymerically bound, and polyacrylates. Resists of this type are relatively insensitive. If the principle of photocatalysis is used in negative systems, distinction is made between resists which can be photopolymerized by means of free radicals and resists in which crosslinking occurs cationically by addition, substitution or condensation as a result of photoinduced reactions. The former have considerable disadvantages with respect to the imaging quality. They are therefore only of limited suitability for applications in the submicron region.
The use of acid-curable resins in photoresist formulations is long known. Thus, for example, U.S. Pat. No. 3,692,560 discloses that halogen-containing benzophenones in combination with melamine resins or urea-formaldehyde resins can be employed as UV resists. DE-A-2,718,259 (=U.S. Pat. No. 4,189,323) describes the use of halogenated derivatives of s-triazine as photolytically activatable acid donors for positive and negative systems, for example for acid-curable urea-formaldehyde, melamine-formaldehyde and phenol-formaldehyde resins (column 5). Examples of formulations of this type are also given in Vollenbroek et al, Microelectronic Engineering 6, 467 (1987).
Photolytically activatable acid formers which can be used also include onium salts, such as diphenyliodonium salts of non-nucleophilic acids, for example of HSbF.sub.6, HAsF.sub.6 or HPF.sub.6. DE-A 2,730,725 (=U.S. Pat. No. 4,193,799) outlines the use of initiators of this type in resist formulations with epoxides as acid-curable materials. A general overview on the use of onium salts in acid-curable systems is given in J. V. Crivello, Polym. Eng. Sci. 23, 953 (1983).
EP-A-0,164,248 describes photocurable mixtures based on acid-curable resins and diazonaphthoquinones or o-nitro-benzoic acid derivatives.
The use of halogenated compounds, such as DDT or gammexan, which are virtually non-absorbent above 299 nm, as photoinitiators in acid-curable negative photoresists is described in EP-A-0,232,972.
Chlorine- and bromine-containing compounds in photo-sensitive materials also include hexabromodimethyl sulfoxide in photographic materials (DE-A 1,572,089=GB-A 1,163,324), 2,2-dibromo-2-(phenylsulfonyl)acetophenone and related compounds as polymerization initiators (DE-A 1,817,540=U.S. Pat. No. 3,615,455), trichloromethyl-substituted benzophenones as polymerization initiators (DE-A-1,949,010=U.S. Pat. No. 3,686,084), haloalkylbenzoxazoles, -benzimidazoles and -benzothiazoles as polymerization initiators (U.S. Pat. No. 3,912,606) and side group-halogenated acetophenones as photocatalysts for acid-curable surface coatings (DE-A 2,317,846).
It is common to the systems described that the initiator compounds do not participate in the actual crosslinking and that--with the exception of the onium salts--they are not soluble in the aqueous alkaline developers themselves. This results in drastically reduced development rates at the points at which the developer is intended to remove the unexposed resist layer. The amount of initiator compound which can be added--and thus the sensitivity of the finished resist--is thereby significantly limited. The use of onium salts, including the initiators described in EP-A-0,232,972 and known as crop-protection agents, is, in addition, also unacceptable for physiological reasons.
It has therefore been proposed, in accordance with the earlier German Patent Application P 38 21 585.3, to use, as alkali-soluble initiator compounds, compounds containing aromatically bound chlorine or bromine which have a pK.sub.a value of less than 12. Although the compounds mentioned exhibit good sensitivities on irradiation with synchrotron radiation, the sensitivities which can be achieved using them on irradiation with UV radiation are, however, relatively low.